1. Field of the Invention
The present invention relates to a display driving apparatus which drives a liquid crystal display panel and a display apparatus using the display driving apparatus, particularly to a display driving apparatus which drives an active matrix liquid crystal display panel.
2. Description of the Related Art
In recent years, display apparatuses having liquid crystal display panels have become increasingly widespread, in such products as digital videos and still cameras, portable phones, and personal digital assistants (PDAs), to display characters and/or images. Liquid crystal display panels are also used as display apparatuses for information terminals, such as computer displays and video monitors, replacing the conventional cathode ray tubes (CRTs).
As the liquid crystal display panel for the above-described use, an active matrix liquid crystal display panel (hereinafter referred to as a TFT-LCD) has been frequently used in which a relatively high image quality is obtained and a thin film transistor (TFT) is used as a switching device.
A major constituent of a conventional display apparatus in which a TFT-LCD is used will next be described with reference to the drawings.
The TFT-LCD is a display in which a TFT for selectively applying a voltage to each liquid crystal display pixel and the liquid crystal display pixels are arranged in a matrix form on a glass substrate.
FIG. 11 shows an equivalent circuit of a liquid crystal display pixel 100 in the TFT-LCD. As shown in FIG. 11, the liquid crystal display pixel 100 includes: a TFT which is disposed in each intersection of a gate line GL extended in a row direction and a data line DL extended in a column direction and in which a gate electrode G is connected to the gate line GL and a source electrode S is connected to the data line DL; a pixel electrode connected to a drain electrode D of the TFT; a liquid crystal display pixel capacitance CLC including a liquid crystal held by an opposed electrode 1 disposed opposite to the pixel electrode; and an auxiliary capacitance CS including an insulating film held between the pixel electrode and auxiliary capacitance electrode 2. In the TFT-LCD, a plurality of liquid crystal display pixels 100 are arranged/constituted in the matrix form. Moreover, a common electrode VCOM is connected in common to the opposed electrode 1 and auxiliary capacitance electrode 2 of each liquid crystal display pixel 100.
FIGS. 12A to 12D show one example of a timing chart of a signal waveform for driving the TFT-LCD.
In FIG. 12A, VG is a waveform showing a potential of the gate line GL, and is a scanning signal. In FIG. 12B, VS is a waveform showing a potential of the data line DL, and is a voltage corresponding to a display data signal, and its center voltage is set to VSDC. These signals VG, VS are applied to the gate electrode G and source electrode S of each TFT.
In FIG. 12C, VCOM is a waveform showing a potential of the opposed electrode 1 and auxiliary capacitance electrode 2 connected to the common electrode VCOM, and its center voltage is set to VCOMDC. When a direct-current voltage continues to be applied to the liquid crystal, the liquid crystal is deteriorated. Therefore, for VS and VCOM, for example, polarity is reversed, and each frame is driven in reverse.
FIG. 12D shows a change of a voltage VLC applied to the liquid crystal capacitance CLC of the liquid crystal display pixel 100.
As shown in FIG. 12D, when the potential of the gate line GL reaches a “Hi” level in a time T1 of a first frame, and thus the TFT turns “on”, the potential of the pixel electrode becomes equal to the potential VS of the data line DL. Thereby, a voltage of a difference between the potential applied to the common electrode VCOM and the potential VS of the data line DL is applied to the liquid crystal capacitance CLC.
In time T2, the potential of the gate line GL is at a “Low” level, and the TFT turns “off”. Thereby, a charge applied to the liquid crystal capacitance CLC is held in the time T1. However, a potential change in a moment in which the potential of the gate line GL is brought into the “Low” level acts in a direction in which the potential of the pixel electrode is lowered via a gate-drain parasitic capacitance CGD of the TFT, and the voltage VLC applied to the liquid crystal capacitance CLC drops by a field through voltage ΔV described later.
In a second frame, the potential VS of the data line DL and the potential VCOM of the common electrode VCOM are reversed, the potential of the gate line GL reaches the “Hi” level in time T3, and thereby the TFT turns “on”. Then, the potential of the pixel electrode becomes equal to the potential VS of the data line DL, and the voltage of the difference between the voltage applied to the common electrode VCOM and the potential VS of the data line DL is applied to the liquid crystal capacitance CLC.
Similarly as the time T2, in time T4, the potential of the gate line GL is brought into the “Low” level, thereby the TFT turns “off”, and the electric charges charged into the liquid crystal capacitance CLC are held in the time T3. Moreover, the potential change at the moment in which the potential of the gate line GL is brought into the “Low” level exerts an influence via the gate-drain parasitic capacitance CGD of the TFT, and the voltage VLC applied to the liquid crystal capacitance CLC drops by the field through voltage ΔV. Thereafter, the TFT turns “off” and thereby the electric charges charged into the liquid crystal capacitance CLC are held.
The field through voltage ΔV is represented as follows.ΔV=ΔVG×(CGD/(CGD+CLC+CS))  (1)
Here, ΔVG denotes a change amount of the potential of the gate line, CGD denotes a gate-drain parasitic capacitance, CLC denotes a liquid crystal capacitance of a pixel electrode portion, and CS denotes an auxiliary capacitance.
As shown in FIG. 12D, when a fluctuation of the field through voltage ΔV is generated in the voltage VLC applied to the liquid crystal capacitance CLC, the waveform of the voltage VLC becomes a positive/negative asymmetric waveform with respect to the voltage VCOM. A difference is generated in positive and negative charge amounts held by the liquid crystal capacitance CLC, and thus a direct-current voltage component is generated.
Thereby, flicker (blinking) is generated. Moreover, when the direct-current voltage is applied to the liquid crystal, seizing occurs, and display quality is deteriorated.
Further, when the direct-current voltage is applied to the liquid crystal, the liquid crystal is deteriorated, and reliability of the liquid crystal drops.
To solve the above-described problem, for example, the center voltage VSDC of the potential VS of the data line DL has heretofore been set to be higher by about ΔV. The positive and negative charge amounts generated by the voltage VLC applied to the liquid crystal capacitance CLC and held by the liquid crystal capacitance CLC are adjusted so as to be substantially the same. Thereby, the direct-current voltage component is reduced, the generation of the flicker is suppressed, and the occurrence of seizing and the deterioration of the liquid crystal are inhibited.
However, the liquid crystal capacitance CLC is not constant with respect to the voltage VLC applied to the liquid crystal. FIG. 13 shows one example of change characteristics of a dielectric constant ∈r of the liquid crystal with respect to the applied voltage VLC. As shown in FIG. 13, in general, the dielectric constant ∈r of the liquid crystal has characteristics that the constant increases with an increase of the applied voltage VLC.
Here, the liquid crystal capacitance CLC is represented as follows.CLC=∈0*∈r*S/d
Therefore, the value of the liquid crystal capacitance CLC also changes in accordance with the applied voltage VLC, and increases with the increase of the applied voltage VLC. Here, S denotes a pixel electrode area, d denotes a cell gap, and ∈0* denotes a vacuum permittivity.
Here, since the voltage VLC applied to the liquid crystal is a voltage based on the potential VS of the data line DL, and the potential VS of the data line DL is a voltage corresponding to the display data signal, the voltage VLC is not constant, and changes in accordance with the display data signal.
That is, since the liquid crystal capacitance CLC changes in accordance with the applied voltage VLC, the field through voltage ΔV also changes in accordance with the applied voltage VLC as represented by the equation (1). Here, a change amount of ΔV by the applied voltage VLC is denoted by ΔΔV.
Therefore, the center voltage VSDC of the data line DL is adjusted in accordance with the state of the applied voltage VLC having a certain value (e.g., maximum voltage). Thereby, in this state, it is considered that the positive and negative charge amounts generated by the voltage VLC and held thereby are adjusted so as to be substantially the same, and set so as to eliminate the direct-current voltage component. However, as described above, the applied voltage VLC is the voltage corresponding to the display data signal, and always changes. Accordingly, the field through voltage ΔV also changes. Therefore, when the applied voltage VLC changes, the positive and negative charge amounts held by the liquid crystal capacitance CLC change. Therefore, the positive and negative charge amounts held by the liquid crystal capacitance CLC cannot be adjusted so as to be constantly the same.
To solve this problem, the auxiliary capacitance CS has heretofore been set to be relatively large so as to reduce the magnitude of the field through voltage ΔV, so that the influence of the change of the liquid crystal capacitance CLC is reduced.
However, to increase the auxiliary capacitance CS, an area of the electrode forming the capacitance CS has to be increased, and thereby an open area ratio drops. Therefore, the display quality is deteriorated, or a luminance of a backlight has to be increased. This causes a problem that power consumption increases.
Furthermore, in recent years, to reduce the apparatus driven by a battery or reduce the power consumption, a driving voltage has been lowered. This increases the use of a low-voltage liquid crystal which operates at a low voltage. In this case, since the voltage applied to the liquid crystal drops, the liquid crystal capacitance decreases, and the field through voltage ΔV tends to be further large. Therefore, the influence of the change of the field through voltage ΔV in accordance with the applied voltage VLC increases, and the flicker and seizing increase. This raises a problem that the display quality is greatly deteriorated.